1. Field of the Invention
The present invention relates to an antenna for a tire pressure information sending apparatus which detects air pressure of a tire and receives/sends information on the tire pressure by radio waves, to a tire pressure information sending apparatus provided with the antenna, and a wheel with the tire pressure information sending apparatus.
2. Description of the Related Art
For safe running of a vehicle, it has become regarded as important to keep air pressure of a tire appropriate, and development working for establishment of regulations that make it obligatory to install an apparatus for monitoring air pressure of a tire in a vehicle has been advanced mainly in Europe and America.
A tire pressure information sending apparatus as described above is provided on a tire side with a pressure sensor that detects air pressure of the tire, and a sending circuit and an antenna that send by radio waves information on the air pressure of the tire detected by the pressure sensor, and on a vehicle side, with a receiving antenna that receives the radio waves, and a receiving unit that informs a driver driving the vehicle of the pressure information.
Japanese Unexamined Patent Publication No. JP-A 2003-347811 proposes that a loop antenna 18 having a radiation electrode having a loop surface at a space of 2 mm above a principal surface 4a of a circuit board 4 on which the sending circuit is mounted as shown in FIG. 8A as an antenna that sends the air pressure information from the tire pressure information sending apparatus is used and the circuit board 4 on which the loop antenna 18 is packaged is attached onto a wheel rim. As shown in FIG. 8B, on the assumption that a direction of an electric current that flows along the loop antenna 18 is expressed by a vector, a loop surface 9 formed by the vector is disposed so as to be in parallel to the circuit board 4. Thereby, it is possible to efficiently propagate radio waves radiated in a tire hollow region.
A tire pressure information sending apparatus is attached to a tire and used in an environment of high-speed rotation. Accordingly it is desired that it is as small in size and light in weight as possible. On the other hand, it is desirable that the antenna is large in order to send radio waves with high efficiency. In general, as an antenna becomes small in size, radiation efficiency of the antenna lowers. To such conflicting demands, it is necessary to make an optimum design in terms of strength and electricity.
The conventional antenna for the air pressure information sending apparatus as shown in FIG. 8A is strong in terms of strength as the loop antenna 18 has a low-profile structure. As shown in FIG. 8B, in the case of showing the direction of the electric current that flows along the loop antenna 18 with the vector and showing a normal direction of the loop surface 9 formed by the vector with a dot-line vector, the normal direction of the loop surface 9 substantially coincides with a normal direction of the circuit board 4, that is, the loop surface 9 is substantially parallel to the circuit board 4. Thereby, an induction electric current generated to the wheel rim for placing the circuit board 4 on which the antenna 18 is mounted is generated in the opposite direction to the electric current that flows along the loop antenna 18. Therefore, radiation radio waves such that the induction electric current of the wheel rim offsets the radio waves radiated by the loop antenna 18 are generated. Consequently, there is a problem such that a radiation resistance becomes significantly small.